This invention relates to a distributor type fuel injection pump for internal combustion engines, and more particularly to a fuel injection pump of this type which can be adapted for use with internal combustion engines with a wide range of the number of cylinders.
In a distributor type fuel injection pump for Diesel engines, it is necessary to vary the fuel injection timing in dependence upon the rotational speed of the engine so as to inject fuel into each engine cylinder at an appropriate time relative to the position of the piston in the cylinder. To this end, the conventional fuel injection pump is provided with an injection timing control device generally called "a timer". A distributor type fuel injection pump provided with such injection timing control device generally includes a drive shaft arranged for rotation at speeds dependent upon the engine rotational speed, a pumping plunger rotatatively coupled to the drive shaft, a cam disc secured to one end of the plunger, a roller holder carrying a plurality of rollers disposed in urging contact with the cam disc and allowed to rotate through a limited angle, and a timer piston drivingly coupled to the roller holder and actuatable in response to fuel oil pressure variable in response to the engine rotational speed. The fuel injection pump operates such that when the drive shaft rotates at speeds dependent upon the engine rotational speed, the plunger is made to rotate and reciprocate at the same time due to the action of the mutually engaging cam disc and rollers, while simultaneously a change in the fuel oil pressure dependent upon the engine rotational speed causes displacement of the timer piston, which in turn causes circumferential displacement of the roller holder, resulting in a change in the position of contact between the rollers on the roller holder and the cam disc so that the acting point of the plunger changes with respect to the circumferential phase of the drive shaft to thereby control the fuel injection timing.
It is a general tendency in recent years for Diesel engines to become more compact in size. For instance, Diesel engines having a total stroke volume of the order of 1,000 cc are under development. In many cases, such compact Diesel engines are designed with three cylinders, depending upon their use. Also fuel injection pumps to be applied to such three-cylinder Diesel engines have to be designed as the three-cylinder type.
However, conventional distributor type fuel injection pumps are very difficult to convert into the three-cylinder type, owing to its structural handicap. That is, to convert a fuel injection pump of this type into the three-cylinder type, three rollers, which are to engage with the cam disc, have to be arranged on the roller holder at circumferentially equal intervals, i.e. intervals of 120 degrees. However, the roller holder has its upper edge portion formed with a large cut for avoiding interference of the roller holder with its adjacent parts. Further, the roller holder has its lower central edge portion occupied by a connecting lever which couples the roller holder with the timer piston. Therefore, it is impossible to arrange one of the three rollers either at the upper edge portion of the roller holder or at the lower central edge portion of same.
In view of this structural handicap, it has conventionally been employed to convert a distributor type fuel injection pump for six-cylinder type engines into the three-cylinder type in such a manner that a communication groove or cutting is formed in tip of the plunger, which extends between adjacent ones of every other pair of six suction slits formed in the same tip. However, this converted fuel injection pump has various disadvantages such as a limited choice in designing the camming surface of the cam disc disposed opposite the rollers, a limited maximum advance in the fuel injection timing, and a too narrow distance between the adjacent suction slits in the plunger, leading to degradated airtightness between the plunger and the plunger barrel.
Under the circumstances, keenly desired is the appearance of a distributor type fuel injection pump for exclusive use with a three-cylinder engine.